On the Coupling between Ionic Conduction and Dipolar Relaxation in Deep Eutectic Solvents: Influence of Hydration and Glassy Dynamics

TL;DR

This study investigates how hydration levels influence ionic conduction and dipolar relaxation in deep eutectic solvents, revealing a transition from coupled to decoupled dynamics and glassy behavior across different physical states.

Contribution

It provides new insights into the coupling between ionic and dipolar dynamics in DES-water mixtures, highlighting the effects of hydration and glassy states on transport properties.

Findings

Coupling between ionic conductivity and dipolar relaxation depends on water content.

Transition from coupled to decoupled dynamics occurs around 40 wt % water.

Glass-like behavior observed in various physical states of DES solutions.

Abstract

We have studied the ionic conductivity and the dipolar reorientational dynamics of aqueous solutions of a prototypical deep eutectic solvent (DES), ethaline, by using dielectric spectroscopy on a broad range of frequency (MHz-Hz) and for temperatures ranging from 128 to 283 K. The fraction of water in the DES was varied systematically to cover different regimes, starting from pure DES and its water-in-DES mixtures to the diluted electrolyte solutions. Depending on these parameters, different physical states were examined, including low viscosity liquid, supercooled viscous liquid, amorphous solid and freeze-concentrated solution. The ionic conductivity and the reorientational relaxation both exhibited characteristic features of glassy dynamics that could be quantified from the deviation from Arrhenius temperature dependence and non-exponential decay of the relaxation function. A transition occurred between the water-in-DES regime, (< 40 wt %),where the dipolar relaxation and ionic conductivity remained inversely proportional to each other, and the DES-in-water regime, (> 40 wt %), where a clear rotation-translation decoupling was observed. This suggests that for low water content, on the timescale covered by this study (~10-6 s to 1 s), the rotational and transport properties of ethaline aqueous solutions obey classical hydrodynamic scaling despite these systems being presumably spatially microheterogeneous. A fractional scaling is observed in the DES-in-water regime, due to the formation of a maximally freeze-concentrated DES aqueous solution coexisting with frozen water domains at sub-ambient temperature.